In computer systems, information is frequently stored in a magnetic film on the surface of a hard or soft disk. The information is stored in concentric tracks in the magnetic film, and it is written to or read from the film by means of a magnetic head or transducer. When storing or retrieving data, the magnetic head rides on a thin laminar boundary layer of air over the rapidly rotating disk, thereby avoiding direct contact with the magnetic surface.
On most disk drives, the magnetic head or transducer is mounted near the end of a member commonly referred to as an actuator. Two configurations of actuators, linear and rotary, have been widely used. In the linear configuration, the actuator is mounted with the magnetic head pointing directly toward the center of the disk and the actuator moves linearly along a radial line to position the magnetic head at a desired position above the magnetic surface of the disk. In the rotary configuration, the actuator rotates about a pivot point near the circumference of the disk, with the magnetic head swinging so as to define an arc over the surface of the disk.
Two further categories of disk drives are defined by the position of the read/write head when the drive is not operating. In "dynamic loading" drives, the head is withdrawn to a position away from the disk (typically on a ramp), whereas in "contact start/stop" (CSS) drives, the head is moved to a "park" position, that is, a position on a nondata zone of the disk (typically near the center) which is reserved for take-offs and landings and resting when the CSS drive is not operating. When the drive is not operating, it is important that the head be restrained on its ramp or other restraining structure if the drive is a dynamic loading type, and that it be restrained in its "park" position on the surface of the disk if the drive is a CSS type. Any abnormal contact between the head and the disk may create a stiction or adhesion or may otherwise damage the head and/or the disk.
Several mechanisms have been proposed to lock the actuator in its proper position when the drive is not in operation. In some drives, the actuator becomes engaged to a passive magnetic or spring-loaded latch when the drive is turned off, the holding force of the latch being overcome by the actuator motor when the drive is turned on again. These mechanisms are vulnerable to becoming disengaged and releasing the actuator if the computer is subjected to a shock force, for example by being bumped or dropped, while not in operation.
Other protective mechanisms rely on a spring-loaded latch and solenoid, the solenoid allowing the spring-loaded latch to restrain the actuator when the power is off and being energized so as to release the actuator when the power is on. While such mechanisms do provide some measure of protection against shock forces when the drive is not operating, solenoid latches tend to be expensive and unreliable, and they consume power while the drive is operating. Moreover, since a very weak spring must be used to avoid the need for a large power-consuming solenoid, even a solenoid latch may be disengaged if the computer is subjected to a strong external shock force. Examples of such mechanisms are disclosed in U.S. Pat. No. 4,716,480, issued Dec. 29, 1987 to Wiens et al., and U.S. Pat. No. 4,725,907, issued Feb. 16, 1988 to Jue, both of which operate with linear rather than rotary actuators.
Rotary actuators are particularly vulnerable to rotational shocks and acceleration. Since a rotary actuator can be designed so that it is substantially balanced with respect to its pivot point, a purely translational shock will operate equally on both ends and will not cause the actuator to move with respect to the rest of the disk drive. Any small imbalances that are due to typical manufacturing variations will not normally create an inertial force large enough to overcome a passive latching mechanism. On the other hand, it is critical to provide protection against the inertial forces arising from rotational shocks, since these may easily cause the rotary actuator to swing about its pivot point, thereby bringing the magnetic head into unwanted contact with the disk. This need has become all the more pressing with the advent of laptop and even smaller computers. These computers operate in a particularly severe environment, and they may readily be subjected to strong rotational forces as they are jarred, bumped and sometimes dropped when being carried about or otherwise not in use.